1. Field of the Invention
The present invention relates generally to anti-reflection and ultraviolet radiation blocking coatings, and more particularly to such a coating utilizing zinc oxide.
There are several areas where it is desirable to have a clear, colorless, transparent surface that is anti-reflective and rejects ultraviolet (UV) radiation. For instance, low reflection, UV blocking glass may be utilized in glazings for framing and conservation of fine art and photographic works. Such glass could also be used for vision correcting lenses. The range of applications of this type of glass would be more extensive if the anti-reflection and UV rejection properties could be imparted to the glass at a reasonable cost.
Additionally, it is desirable to achieve the anti-reflection and UV blocking properties without imparting color to the transparent surface. It also advantageous to provide such properties with a coating on only one side of the transparent surface. For economic considerations, it is preferred that the coating be deposited on a large scale by DC magnetron sputtering equipment of the type typically used for coating architectural glass.
All UV radiation is damaging to the eyes, upholstery, and works of art such as paintings and photographs. The radiation becomes less damaging at longer wavelengths, but over extended periods even the long wavelengths, i.e. light in the violet region of the visible spectrum, will cause fading of certain pigments. Likewise, shorter exposures to the shorter UV wavelengths cause damage. Accordingly, it is desirable to extend the UV blocking region as far as possible towards the visible spectrum, definitely as far as wavelengths of 380 nanometers (nm), without compromising visible light performance.
2. Description of the Related Art
Multilayer Anti-Reflection Coatings
There are many patents covering broad-band, multilayer, anti-reflection coatings. These patents cover variations of the three or four basic film structure, discussed in detail below, and they are typically directed to using mixtures of materials or substituting two or more films for one film to avoid using materials with a specific refractive index (n). See, e.g., U.S. Pat. Nos. 3,432,225; 3,565,509; and 4,260,222.
Other patents concern the addition of films to the basic structure or substitution of two or more films for a film in the basic structure for purposes of extending the effective spectral range. See, e.g., U.S. Pat. Nos. 3,781,090 and 3,960,441. One patent, U.S. Pat. No. 4,422,721, is directed to the incorporation of transparent conductive oxide films, such as indium tin oxide, into the basic structure for purposes of heating the coated object, bleeding-off static electricity, and preventing penetration of electromagnetic radiation at radio and microwave frequencies.
UV Absorbing Glass
Most common plate glasses and optical glasses absorb UV radiation to some extent. Common soda lime glass, for example, begins to absorb in the violet region of the spectrum but the absorption increases only gradually. Glass utilized for art glazings, frequently 2 millimeters (mm) thick, does not absorb UV radiation completely until 320 nm.
Certain metal oxides can be incorporated into glass formulations to impart UV blocking properties. The most-utilized materials are cerium oxide, titanium oxide, neodymium oxide, and erbium oxide. Such formulations are varied and complex, but the overriding problems in designing the formulations are similar.
Not all oxides can be accommodated in a glass matrix. Some oxides may be accommodated, but only in certain proportions, and/or in the presence of other components which may be undesirable for the purpose for which the glass is to be used. Invariably, materials suitable for effectively blocking long wavelength UV radiation, e.g. greater than 400 nm, will also absorb visible radiation, and thus add color. This happens either as a result of their effect on the glass structure or due to the presence impurities in the oxides. This color may be neutralized by the addition of other components but at a loss of visible light transmission. In all instances, the increase in absorption with decreasing wavelengths is sufficiently gradual that complete blocking at wavelengths longer than 350 nm can not be accomplished without absorbing the blue portion of the visible spectrum and imparting a yellow transmission color to glass. U.S. Pat. Nos. 1,536,919; 1,634,182; 3,499,775; and 4,701,425 relate to the problems described above.
Plastics and Resins
The most-effective UV blocking materials produced in recent years have been specially-formulated plastics and resins. The typical performance of two different plastic materials, produced by the Rohm and Hass Company, Philadelphia, Pa., and designated UF 3 and UF 4, is shown in FIG. 1. The shorter wavelength blocking material UF 4 imparts no perceptible tint to the transmitted light. The longer wavelength blocking material UF 3 imparts a slight yellow tint.
Resin formulations are typically based on silicon siloxane resins containing appropriate additives. They are designed to be coated on glass by spinning or roll coating and then are heat cured. The performance of such products compares with the plastic materials UF 3 and 4.
The plastic and resin products have a steeper absorption edge, i.e. the change from UV blocking to transmitting, which is desirable, than UV absorption glass products. Both plastics and resins possess a basic disadvantage, however, in that they are relatively soft and thus will deteriorate under repeated cleaning. Therefore, they must be protected by placing them in assemblies laminated with glass, or by providing them with hard overcoatings. These measures add significantly to the cost of the finished product.
Multilayer UV Reflectors
It also possible to reject UV radiation using a UV reflector formed from a stack of alternating high and low refractive index UV transparent dielectric materials, for example titanium oxide (TiO.sub.2) and silicon oxide (SiO.sub.2). To reproduce the blocking level and the absorbing edge slope of the UF 4 material, e.g., requires at least seventeen individual films.
While the transmission of visible light through a UV reflector may be optimized by using the well-known long wavelength pass filter structure, a truly low reflection, typically less than 0.5%, can only be realized over a narrow portion of the visible spectrum. Additionally, it is not clear whether such a structure could be integrated into a broad-band, anti-reflection multilayer structure. Even were such a solution possible, there still remains the issues of economics and the compatibility with large scale, in-line sputtering operations.
UV Absorption by Thin Film Materials
Certain of the oxide materials discussed above in connection with UV absorbing glass could also absorb UV radiation in their pure form. This is certainly true for titanium oxide. This is a high refractive index material useful in certain types of multilayer, anti-reflection coatings, as will be discussed later.
Titanium oxide is a strong absorber of UV radiation, at least to the extent that significant absorption begins at a wavelength of about 450 nm. However, the increase in absorption with decreasing wavelength is very gradual. A typical value at 380 nm for the extinction coefficient k, the imaginary part of the so-called complex refractive index (n-ik) of UV absorbing materials, is 0.005. This is equivalent to an absorption coefficient of 16.times.103 cm.sup.-1, which means that a film would need to be on the order of 10,000 nm thick to absorb 99% of the light at this wavelength. To put this in perspective, a film of TiO.sub.2 with an optical thickness of one-half wavelength at 520 nm is approximately 110 nm thick. This is typical of the thickness of this material when used as a component multilayer, anti-reflection coatings.
Zinc Oxide
Zinc oxide (ZnO) is a material which is rarely, if ever, used in multilayer film structures deposited by thermal evaporation. This is primarily because of the availability of other materials with a higher refractive index which are just as easy to deposit and more durable. In the architectural glass coating field, however, where films are deposited by DC reactive sputtering from metal targets, ZnO is commonly used because of the high deposition rates that can be achieved.
The UV absorption edge of DC reactively-sputtered ZnO films is very steep with the extinction coefficient k rising very rapidly in the region from 400 nm to 380 nm. Following the sharp increase in absorption with decreasing wavelength, k becomes uniform at a value of approximately 0.4 at wavelengths between 370 and 300 nm. At this value, a film at least about 250 nm thick would cause transmission therethrough to fall below 99 percent at wavelengths for which the k value has reached a maximum. A ZnO film 250 nm thick is optically twice as thick as the thickest films used in conventional anti-reflection coating structures.
The refractive index of reactively-sputtered ZnO films in the visible region of the spectrum lie in the range between about 1.85 and 1.91. As will be shown, this value coupled with the thickness necessary to provide adequate UV absorption makes it impossible to incorporate them as film components in broad-band, anti-reflection film structures while providing the required level of UV absorption.